In recent years, blow molded containers of thermoplastic resins have been developed in the form of containers for a variety of drinks and foods. Now, single-layer PET bottles comprising polyethylene terephthalate (PET) are commonly used as such blow molded containers.
However, the PET bottles are found to be less than satisfactory for containers for stuffs sensitive to oxygen, and so are required to have gas barrier properties in general, and carbonic acid gas barrier properties for carbonated beverages as well. When liquid food stuffs such as jams, jellies and fruit sauces are packed in blow molded containers, this is generally achieved by hot-filling. Also carbonated fruit juices or lactic drinks, etc. are heat sterilized by hot-water showering upon packed in blow molded containers. Thus, the blow molded containers are required to have properties high enough to stand up to heat and pressure. To sum up, the blow molded containers must possess high gas barrier properties and heat resistance enough to be resistant to hot-filling.
To improve the gas barrier properties of blow molded containers, some processes have already been proposed, wherein a multilayer container with a gas barrier resin layer provided in an intermediate layer is formed by blow molding.
JP-A 56-64839 proposes a multilayer container production process wherein a container precursor having a multilayer structure comprising an outer PET layer and an inner PET layer with an intermediate layer formed of a methaxylene group-containing polyamide resin, and then subjected to biaxial stretch blow molding.
JP-A 57-128516 comes up with a blow molded container having a multilayer structure comprising at least two thermoplastic resins, wherein at least a thin portion of its body has a three- or multi-layer structure, at least the end of its opening has a single structure, and at least the thin portion of its body has a biaxially oriented multilayer structure. This publication describes that PET is used as the thermoplastic resin that forms the inner and outer layers and the end of the opening, and EVOH or a methaxylene group-containing polyamide resin is used for the intermediate layer.
JP-A 62-199425 discloses a process for producing a biaxially stretched container, wherein a multilayer pre-molding article comprising an inner PET layer and an outer PET layer with at least one intermediate layer comprising a gas barrier resin is subjected to biaxially stretch blow molding in a mold held at a thermally fixed temperature and the blow molded article is heat treated, after which the blow molded article is cooled and removed from the mold. An example where EVOH and a xylene group-containing polyamide resin are used is set forth in that publication.
Since the 1990s, multilayer containers having a structure of, for instance, PET/EVOH/PET or PET/MXD6/PET layer construction and obtained by co-injection stretch blow molding, for instance, in container forms for beers or wines, have already been commercially put on the market.
Since EVOH has a melting point close to a thermal decomposition temperature, a high melt viscosity, etc., however, it is considerably difficult to subject EVOH in combination with PET to co-injection stretch blow molding. More exactly, it is difficult to determine conditions for co-injection stretch blow molding of two such resins, because there is a large difference in proper molding temperature between them. Upon high temperature injection, for instance, the melting temperature becomes high by crosslinking (gelation) of EVOH, ending up with instable melt flows. This inevitably causes the intermediate layer or the EVOH layer to vary largely in thickness, and implanting height (height from the bottom of a bottle to the tip of the EVOH layer) to become unsatisfactory or vary, resulting in lack of gas barrier properties and defective appearance.
On the other hand, MXD6 nylon that is a typical methaxylene group-containing polyamide resin possesses very excellent co-injection capability when used in combination with PET, because of having a melting point close to that of PET. In addition, since both resins have an approximate glass transition temperature, it is easy to determine a proper molding temperature for stretch blow molding. However, MXD6 nylon is less satisfactory than EVOH in terms of gas barrier properties, and so a blow molded container with an MXD6 nylon intermediate layer does not lend itself to applications where oxygen barrier properties are needed over an extended period of time or high gas barrier properties are demanded.
JP-A 61-47337 discloses a process for producing a multilayer bottle of layer construction comprising PET/HBR/PET by co-injection stretch blow molding, wherein a resin obtained by polycondensation of dimethyl terephthalate and ethylene glycol is mixed with a resin obtained by ring-opening of glycolide at various mixing ratios, the mixture is melt polymerized to obtain a polymer (HBR) having high gas barrier properties, and the HBR and PET are subjected to co-injection stretch blow molding. However, the oxygen gas barrier properties of the HBR set forth in that publication is not sufficiently elevated, say, on the order of 2.3×10−13 cm3·cm/cm2·sec·cmHg at best, as estimated by an oxygen permeability coefficient (PO2) measured at 25° C. That publication says nothing specific to a heat-resistant blow molded container having heat resistance enough to stand up to hot-filling.
JP-A 10-138371 discloses a gas barrier, multilayer hollow container having a multilayer wall construction wherein a thermoplastic resin layer is laminated on at least one side of a layer formed of polyglycolic acid. That publication sets forth a process for producing a multilayer hollow container by multilayer extrusion blow molding, multilayer injection blow molding or the like, and gives a specific example of producing a multilayer hollow container comprising an inner PET layer and an outer PET layer with a polyglycolic acid intermediate layer interleaved between them via an adhesive layer by means of co-injection stretch blow molding. With such multilayer hollow containers produced by co-injection stretch blow molding as described in that publication, however, there is still much to be desired in terms of co-injection stretch blow molding conditions, gas barrier properties, durability, heat resistance, moldability, etc. More specifically, the problems to be challenged are to enhance durability by reliable embedding of a polyglycolic acid layer that is susceptible to decompose under environmental conditions in a thermoplastic resin layer, improve gas barrier properties, and achieve heat resistance enough to stand up to hot-filling.